Technical Abstract:
The objective was to evaluate the effect of ruminally degraded protein (RPD) source on microbial protein flow measured in the reticulum and duodenum using 15N and in milk, urine and blood using allantoin. Eight early lactation Holstein cows were arranged in a repeated 4x4 Latin square design balanced for carryover effects with 21 d periods. All diets were isoenergetic (1.71 Mcal/kg) and had the same RUP content (5.6%). Cows were fed either a base diet containing 12.8% CP or one of three treatment diets containing 16% CP supplemented with urea, casein or both. Cows were also infused with Cr-mordanted NDF, Co-EDTA and 15NH4 as solid passage, liquid passage and microbial protein markers, respectively. Microbial protein flow was significantly lower for the base diet that for the other diets for all methods to estimate microbial protein. Nitrogen flow from microbial origin through the duodenum in cows fed the base diet was 237 g/d compared with 292 g/d for cows fed the other three diets. Total microbial N leaving the rumen was 227 g/d for cows fed the base diet compared with the average of the other three treatments (322 g/d). Allantoin concentration, an indicator of microbial protein yield, was lower in milk and plasma for cows fed the base diet (130 and 191 'mol/L) than for cows fed the urea/casein diet (267 and 231 'mol/L, P < 0.05). Additionally, allantoin excretion in urine was 81 mmol/d for cows fed the base diet compared with 116 mmol/d for cows fed the urea/casein diet (P < 0.05). These results indicate that by using passage markers, reticulum samples can accurately measure flow of microbial protein to the duodenum, and allantoin was a good indicator of microbial protein flow. Additionally, rumen microbes can use amino acids or non-protein N in equal efficacy and the amount of RDP in the diet, not the source of RDP, is most important in diet formulation.